This application relates to solid bowl decanter centrifuges of the scroll discharge type and more specifically to their use in applications entailing sewage sludge dewatering assisted by the addition of polymeric flocculants.
The final dewatering of combined primary, secondary or digested sewage sludges or mixtures of these may be advantageously carried out using solid bowl decanter centrifuges of the scroll discharge type. The process invariably entails conditioning of the sludge by the addition of a polymeric reagent to promote the aggregation of small primary particles to larger aggregates capable of settling under the action of centrifugal force at a greater rate than the primary particles. One of the major problems associated with the use of flocculants in decanter centrifuges concerns the high local shear forces generated in the zone where the solid/liquid suspension enters the rotating bowl and is accelerated to bowl speed. With some flocculants, if the flocculant is added to the suspension prior to its introduction into the machine, then in the presence of velocity gradients and turbulence in the feed zone any flocs which may have formed will experience shear forces causing them to disrupt. With polymerically bridged flocs this disruption is irreversible.
To overcome this problem the introduction of flocculant directly into the centrifuge bowl pond via a separate feed pipe has been suggested. This effectively overcomes the problem of floc disruption but often mixing of suspension and flocculant and the residence time within the bowl after mixing, is inadequate to promote effective conditioning.
Other methods have been proposed where the feed chamber within the conveyor consists of a conical section to provide gradual acceleration of the feed suspension and flocculant. As a further solution in an effort to overcome the turbulence associated with the addition of flocculant to the suspension in or prior to the feed zone, special high shear resistant flocculants have been developed. With these the turbulence occuring in the region of the feed zone effectively aids mixing and can be beneficial to the subsequent flocculation process.